The present invention relates to a vacuum apparatus. In particular, the vacuum apparatus may be used in a disk recording system, for recording a disk such as a compact disk.
Many systems are known in which information is recorded on a disk-shaped medium and may subsequently be played back. Generally the information is arranged either in substantially circular rings or in a continuous spiral track on the disk. An example of the former arrangement is the magnetic floppy disk or hard disk, where the information is divided into sectors lying in concentric tracks. Examples of the latter arrangement include the conventional gramophone record carrying sound information in analog form in the spiral groove in its surface, the optically read videodisk carrying video information in analog form in a series of pits arranged spirally on the surface (or on an interfacial boundary) of the disk, and the compact optical disk carrying audio or other information in digital form in a series of spirally-arranged pits. The gramophone record, the videodisk and the compact disk are all examples of media available to the consumer which cannot normally be recorded on by the consumer; recording takes place on a master disk which is subsequently replicated by various processes such that the disks bought by the consumer are close copies of the geometry and the information content of the master disk.
In particular, recording onto the master disks for video disks and compact disks is conventionally performed by means of a laser beam incident on the master disk. Alternatively, an electronic beam can be used to irradiate a recording surface of a disk. In this case, a vacuum chamber must be provided in which the electron beam can operate.
The process of recording information on any of these media usually shares in common the fact that the disk or master disk is rotated at a speed typically anywhere between 16⅔ r.p.m. (for some gramophone records) and 1,800 r.p.m. (or even higher for some videodisks) while the point of recording (which may be a magnetic head, a mechanical stylus, or a focussed light beam) is traversed between the inside and outer edge of the disk at a slower rate. Normally it is a requirement of the recording system that the rotational motion of the disk may vary only slowly, if at all; generally this is easily ensured by the inertia of the disk itself, together with that of the mechanism which rotates it. The radial motion of the point of recording on the disk is, however, not so easily controlled. In the case of magnetic disk recording, it is usual that the recording head must move in discrete steps between the separate concentric tracks; by contrast, in the cases of gramophone records, videodisks, or compact disks, the recording head must move continuously relative to the disk in a generally radial direction in order to lay out the information in a spiral track, and it is characteristic of these cases that the smoothness of the radial motion is more important than the absolute accuracy of radial positioning. With a gramophone record, for example, any radial motion having significant energy in the audio frequency band, even if it represents only a small fraction of the average groove spacing, will appear as a corresponding lateral movement of the pickup when the final record copy is played, and this will be audible as a noise superimposed on the recorded audio signal. With videodisks and compact disks there is not only the possibility that any sudden radial motions of the recording head will cause the player to fail to follow the track on the final disk, but also the more serious likelihood that such motions will be dangerous simply because they will result in significant changes in spacing between successive turns of the spiral track. Since this spacing is typically only 1.6-1.7 xcexcm, in the case of Compact Discs and 0.74 xcexcm or less in the case of higher capacity discs, and any reduction in spacing has the effect of increasing the crosstalk between tracks (resulting in interference in the picture from a videodisk, or an increased likelihood of bit errors with a compact disk) it is desirable to maintain a tolerance of at most +0.1 xcexcm in the track spacing, and preferably a much closer tolerance than this, in the case of higher capacity discs of +0.01 xcexcm or less.
To obtain the necessary radial tracking motion, it is usual to move the recording head along a straight line which passes through the axis of the disk, in other words radially. When recording gramophone record masters this is commonly achieved by mounting the recording head on a linear slide or rolling mount and moving it by means of a rotating leadscrew and nut. Satisfactory performance is achieved by careful engineering; the stiffness of the leadscrew drive is great enough to overcome residual friction in the mounting. In videodisk and compact disk mastering (recording) a similar technique may be used, in which the optics which produce the focussed beam are moved over the rotating master disk. To avoid the disadvantage that part of the optics are thus movable while the remainder (owing to the size of the light source, normally a laser) have to be fixed, it is alternatively possible to move the entire turntable (which carries the master disk) together with its rotary bearing along a straight line, using a leadscrew, while the recording head remains fixed.
In long-playing optical videodisks, or optical compact disks used for audio or other data in digital form, a constant linear velocity mode or recording is normally used because it allows the maximum recording time consistent with operation at the optimum linear velocity (which determines the bandwidth of the signal which can be recorded) throughout the recording.
In the system described above, either the recording head or the turntable bearing can be the moving element. However, EP-A-0619042 proposes that the turntable is arranged to rotate on a turntable bearing, and the turntable bearing itself is mounted on a support body via an air bearing so as to be moveable relative to the recording head. A fluid filled dashpot is provided for passive damping of the motion of the air bearing and thus stabilise the motion of the turntable with reference to the frame of the machine itself. Furthermore, the turntable bearing itself is a rotary air bearing.
In its most general terms, the present invention provides a rotatable shaft which passes through the wall of a vacuum chamber and provides an exhaust path for air exhausting from an air bearing within the vacuum chamber.
A first aspect of the present invention may thus provide a vacuum apparatus including
a vacuum chamber,
a rotatable shaft mounted on a support body via a first air bearing, the first air bearing being provided within the vacuum chamber,
a second rotatable shaft extending through the wall of the casing of the vacuum chamber and mounted therein by a second air bearing,
wherein the second shaft is hollow along its axis of rotation and the hollow portion is in communication with at least one air output from the first air bearing for the removal of air exhausting from the first air bearing.
The first air bearing may be enclosed in a second chamber. Using the vacuum apparatus of the invention, air required for the first air bearing can be supplied and can be removed through the second shaft.
In particular, it is envisaged that the vacuum apparatus of the invention be used in a disk recording apparatus. In such an apparatus the first air bearing may support a turntable, the turntable being located within the vacuum chamber. Since the bearing mechanism of the turntable is not exposed to that vacuum, it may comprise a bearing of a kind which cannot be used in vacuum, in particular an air bearing. Means may be provided to restrict leakage of air from the air bearing into the vacuum chamber.
A vacuum pump may be provided to evacuate the vacuum chamber to a pre-determined level. For example, in use, it is expected that the vacuum chamber will be operated at or below about 10xe2x88x925 Torr.
The first air bearing may be arranged to be movable within the vacuum chamber relative to the second air bearing. In the case where the vacuum apparatus of the invention is used in a disk recording system the first air bearing may be arranged to be movable relative to the recording head and in the case in which the first air bearing is provided in a second chamber, the second chamber may be arranged to be movable relative to the recording head by mounting the second chamber on an arm rotatable about the second air bearing passing through a wall of the vacuum chamber which forms the frame of the recording system. In this case, it is particularly advantageous that the second chamber of the turntable drive mechanism communicates with an internal portion of the second shaft, so that air required for the air bearing of the turntable drive mechanism can be supplied to the second chamber, and air exhausting from the drive mechanism can be removed from the second chamber through the internal portion of the second shaft. Thus, the air bearing on which the second shaft turns may provide a very high precision frictionless mechanical connection into the vacuum chamber whilst leaving all the drive mechanism outside, and at the same time may provide a rotatable frictionless high vacuum feed through for cables and pipes to and from the first air bearing and, if present, the second chamber or other mechanism within the vacuum chamber.
For example, it is possible to provide at least one input conduit within the second shaft communicating with an input portion of a second chamber for supplying air to the first air bearing inside the second chamber, and to provide at least one output conduit within the second shaft communicating with an air output from the second chamber for removing air exhausting from the first air bearing.
The present invention may therefore provide an air bearing through a wall of a vacuum chamber to provide both a frictionless mechanical connection into the vacuum chamber and a rotatable frictionless high vacuum feed-through into the vacuum chamber.
Thus, using the apparatus of the invention, a disc can be located on a turntable and processed by an electron beam which operates in a vacuum, the turntable itself being supported by an air bearing within the vacuum chamber. The air bearing may be enclosed within a second chamber within the vacuum chamber, with the drive mechanism located outside both chambers. The electron beam can process the surface of the disc to a higher level of accuracy when using the apparatus of the invention than when using conventional systems, leading to an enhanced recording quality. Such apparatus is applicable, for example, to recordable and erasable discs, including those which carry an embossed pattern of circular or spiral tracks.
One or more vacuum sources may be provided in communication with some or all of the output portions of the first air bearing, via the output conduit(s) of the second shaft. These vacuum sources help to prevent air exhausting from the air bearing from contaminating the vacuum chamber.
For example, in the case of a recording system comprising a vacuum apparatus according to the invention, a first output from the second chamber may be the exhaust region of the first air bearing, and be placed in communication with the atmosphere surrounding the recording system, while a second (xe2x80x9croughingxe2x80x9d) output may be provided between the first output and the sealing means. It has been found that this measure radically reduces the pumping requirement of the pump which evacuates the vacuum chamber.
One or more vacuum sources may also be provided in communication with some or all of the output portions of the second air bearing. The vacuum source may be the same for both air bearings.